I was wondering how to change existing custom neutral makeup, that's normally only for female sims, for example, available also to male sims and to most ages. Does changing a particular property within the package make it accessible despite the different simtypes? That is, can I convert it with just simPE?
Or is the package missing vital information for the game to understand how to translate the textures? So that I'd have to create a fresh recolor in Bodyshop, to convert it properly?

The easiest way is to edit the ages in SimPE (or some other program, like Wardrobe Wrangler or the Outfit Adjuster). No need to make a fresh recolor.

Face textures are essentially identical for all ages (you can use the same ones, anyway - maybe minus teeth for infants), so it's really the easiest thing to convert. You can even make items available for infants and toddlers (neat for natural face details like birth marks, freckles, etc.)

How to edit depends which ages/genders it's already available for. Ideally you only want to add the missing ages/genders. In the two other programs I think you can just select which ages/genders you want, and then save the file. This should add the missing ages/genders to the existing resources.

In SimPE, you find the "Texture overlay XM (XTOL)" resources. They're split into separate ages/genders, so there's one for each. The lines you're editing are the "Age" and "Gender" lines.

You'll first want to see which is which, and if there are more than one (preferably there's at least two, but it helps with more).

Female = 1, male = 2, unisex = 3.
Infant = 20, toddler = 1, child = 2, teen = 4, adult = 8, YA = 40, elder = 10).

You can combine the ones you want to add, so if there's just adult female (age 8, gender 1) and no male adult, you can change the gender to 3.

Combine the younger ages in the lowest age you find (if it's teen, use that, if it's child, use that. If you only find YA and adult, combine teen and below into one = 27 or 7, and set the other to YA through elder = 58). Make sure to not skip over any ages, or you'll get disappearing makeup.

(This only works for age-converting makeup, not clothes or accessories, since those involve meshes)

Thank you!