Adding custom mobs to HLO

[TheYsconator]

I may be overlooking some very obvious button, but how do I add mobs to HLO that are not currently available in packages sold by Lone Wolf?

In my particular case: I've purchased the Bestiary and Core set for PF2, and would like to add some mobs that are not in these packages (something home brewed, or something from PF1 that has not been converted to 2E by Paizo yet). How would I do that? I tried picking something that looks like something I would like to make and changing that, but that has to be done using permanent adjustments, which is a loooot of work for something so seemingly easy. I also looked for a 'custom' or 'other' ancestry, but that does not appear to exist. Am I overlooking something?