Yeah it is possible. By replacing some character from sr4 to a character you want to add. That's easy tho.
So u need to unpack str2_pc packages of the both characters (drag and drop them to ThomasJepp.SaintsRow.ExtractPackfile.exe)
in my case i want to replace gang_female_jill from sr4, to deckers_female_grace from sr3
in my case i want to replace gang_female_jill from sr4, to deckers_female_grace from sr3
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i'm just dragging and dropping files from grace folder to jill folder and deleting jill files (jill have a sim_pc file that grace don't have, so i left original one in the folder)
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Then you need to change file names (in my case from grace to jill. be careful to not delete the "_head" and "_head_lod" lines)
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And then i do the same thing with "high" packages (gang_female_jill_high)
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then build this folders as packfiles using ThomasJepp.SaintsRow.BuildPackfileGUI (use original packfile name (gang_female_jill.str2_pc in my case) and build it into your sr4 folder) and update character_containers.asm_pc (if you don't have it, you can find it in charactes.vpp_pc, just copy it into Saints Row IV folder)
enjoy results
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p.s. my model ended up being glitched as fuck. but don't worry, i did this many times and there was no problems. i guess it's because of original ho model that i was replacing. maybe because of that sim_pc file, idk. but in other cases everything was working well.
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