Android (and iOS) supports the following plurals: "zero" | "one" | "two" | "few" | "many" | "other". It'll be nice to be able to add those forms supported in POEditor.

Custom plural formulas is something we've never implemented, because of the problems that might have come with it (and also, the user level of knowledge it would have implied).
Now, every language we support has its own plural formula, checked/tested/standard. You will find the plural forms you are looking for according to the language you are translating in.
If you really need some special plural forms, you can use a language that fits this exact case (we never met such cases to think seriously about a "feature" like this).

But how to add more than one plural ? Right now, i only got the "one" and "other". My need would be to have a more customizable choice (like 0, 1, few, many, others...). And if it's possible, to be able to manage these attributes myself.

For a term with no plural forms set:
Project --> View or Add Terms -- hover the mouse over the term--> press P --> insert the plural form

That term will have the corresponding plural forms displayed in every new language, so the collaborators can translate each form in their language.