Get young rats (5-7 weeks is best), and of course, get the same sex as the rats you have
Females are usually quite accepting of new rats - they often will "mother" the new babies. If your rats are 5 mos, they're still quite young, and introductions may go relatively quickly.
There is always a small chance that one of the resident rats will NOT accept a new rat. It is not usual, but I'm experiencing this now with my males. So, I recommend introducing new rats in pairs. The rat will always have a friend in quarantine and during the intro process.
Here is a good article to follow:
http://www.ratfanclub.org/newrat.html