I was told that I should post my discovery up on the forums, so here it is. I'll be posting it as a copypasta from Discord. If you have any questions, contact me on Discord at MicroKnightmare#9776.
"In essence, you export peds.ymt from OpenIV as an XML, and edit this line in the items Player_One, Player_Two, and Player_Zero:
But if you import the modified XML into the update.rpf with either OpenIV or Codewalker, it will crash on loading the story mode.
What you need to do is DELETE the XML extension and rename the file as peds.ymt and directly import it back into the RPF with the tool of your choice (I used OpenIV). Afterwards, find a component ped model, such as IG_NATALIA for example, and rename the .YDD, .YTD, .YFT, and .YMT into a player ped, such as player_zero
ig_natalia.ydd -> player_zero.ydd
ig_natalia.ytd -> player_zero.ytd
ig_natalia.yft -> player_zero.yft
ig_natalia.ymt -> player_zero.ymt
Finally add the newly renamed player ped files, in this case player_zero (Michael) into x64v.rpf as well as the dlcpacks 'mppatchesng,' 'patchday3ng,' and 'patchday4ng.'"
The method can be reversed to add a streamed ped to a nonstreamed/component ped.
Hope this helps people wishing to convert Addon Peds to Player Peds for their personal singleplayer story mode runs!