WinME *should* be smart enough to just add itself to WinNT4/2K's boot loader. If you are only running Win2K now, you probably don't see it during boot up, but it's there.

But, I always like to use 'setup /ntldr' to install WIn9x/ME just in case 9x/ME doesn't see the existing install.
The limiting factor will be your C: partition. If it is FAT, you are OK. If it is NTFS, you may have to do some repartitioning, reformatting, and reinstalling. That's why it is typically easier to install Win9x/ME before NT/2K.
-SUO