Using mSL, it is completely possible. Infact a few scripts exist out there that provide this functionality. Some are older, theres an AimIRC which only supports TOC 1.0 (I used it for awhile after updating it to TOC 2.0), but there are a few current MSN/Yahoo protocol handling solutions. DLLs exist as well for protocol handling.
The method is using mSL and a proxy which translates between the protocols. This feature doesn't exist in mIRC natively, it has to be coded using it's internal language (mSL).
I hope this answers your question more diligently.