I'm using a custom character that only has access to the default hairs and Ren's hairs... I'd like to add Soya and some others

Open up the Contruction set, load the esp from whatever custom race you're using and make it the active file, then, load the other mods you want to "import" from, like in your case the esp that only has the hairs (it's more pratical i guess) and the soya ones.

Once they are all loaded you should open Character>Eyes and Character>Races

You should have the hairs you need on the Eyes window, then, choose the desired race and click on the body data tab and you just need to drag the hairs you want to the Hairstyles list
Hope it helped!

(Btw... It can cause some wierd stuff sometimes, like getting duplicated races, which happens a lot to me... xDD)

Worked like a charm... thanks!!!!