It can be opened with almost any text editor, you must shut the game down to reload the file, like the pedstats.dat
There are also two spare slots in the file (265, and 266).
If you wanna add a new ped in, open the file, and find:
#---ADD NEW PEDS HERE
After this add for an example something like this in:
265, eax, eax , CIVMALE, STAT_TOUGH_GUY, man, 1FFF,0, null,1,4,PED_TYPE_GEN,VOICE_GEN_NOVOICE ,VOICE_GEN_NOVOICE
EAX is the model and texture name of the ped in the gta3.img,
you must change that with you're peds name in the gta3.img, other flags can be changed if needed, to do so.