Couple things. First, here is a handy calculator to help you determine how many mice can comfortably be housed in any given container:
http://www.thefunmouse.com/info/cagecalculator.cfm
Please note that when you get a "9.6" number that does NOT mean "9 males 6 females." The "." is a decimal point, and means "nine point six individuals."
Another note about that calculator--I do NOT recommend posting on the associated forum, as it is a forum for PET mice lovers only, and feeder breeders usually get a pretty good lashing. So for your own safety...
Back to adding females to your colony. Mouse fancy breeders do it all the time. It's no biggie, but there are precautions you must take if you wish to do it successfully:
First, quarantine your new mice just as you would a new snake--usual quarantine is three weeks, but you're okay at about two, usually. This is for the same obvious reasons as you quarantine your snakes, except that mice can die a lot more quickly and you can wipe out your entire colony before you even realized there was a problem.
Second, when you are ready to introduce the new mice, take them all out and let them play together in a totally neutral territory (some place that does not smell like either group of mice--a sterilite tub, the top of a table with no way for them to climb down, etc) and under supervision (do not do any of this if you have nursing mothers with babies under four weeks, by the way--you could easily lose the babies and possibly some adults as well).
If they play together fine without too much fussing, you can leave them alone for a bit while you thoroughly clean and disinfect everything in their enclosure. Wheels, toys, etc. Dispose entirely of their bedding. There should be not a single molecule of the existing colony's mouse smell remaining. Use baking soda or bleach or whatever you need to to get the smell completely out.
Put new bedding in the cage. It's best also to add a NEW nestbox, and leave the old one out for now (you can re-introduce it later). Everything about the enclosure should seem totally new to the mice--so put things in different locations than usual and include only toys they're not familiar with (except the wheels, which should be thoroughly de-scented). The new nestbox can be a small cardboard box with a hole cut in a corner, an upside down clay pot with a hole, or whatever you can find that is the right size, as long as it won't seem familiar to your existing colony. Make sure there is only ONE nestbox and that it is large enough for all of them but small enough for them to feel secure. You want them to sleep together, as sleeping together is a major form of bonding for mice (isn't it for all of us?

).
When introducing new mice, I also do not put many toys in the cage for at least a week (the fewer things to fight over, the better--you want them playing with each other, not with their toys). A couple of wheels (mine ALWAYS have wheels, and I do strongly encourage their use, as mice really need to run and exercise to be happy and healthy--I have one wheel for every two mice) and maybe a ladder and a toilet paper tube or two should be sufficient for the first week.
Remember that while snakes live life in the slow lane, mice live it in the fast lane. If after a week they are getting along with few squabbles and sleeping together at least most of the time, then they are practically family and they'll be fine. You can now add back the old nestbox (thoroughly cleaned) and any additional toys you may want to use.
BUT, do keep a close eye on them in their "new" enclosure for the first 24 hours (in fact, I wouldn't leave their room for the first two or three hours). If there is some squeaking and fussing at first it's not a big deal. But if you've got blood or serious biting (ear nicks or hindquarter abrasions), then you may have to separate them and try again in a few days.
Good luck.
Heather