Multi-select Option Set Alternative for Portal

Hello Everybody,

I'm looking for an alternative solution to having multi-select option sets on the dynamics portal.

my scenario is that i have an online application the needs to be filled out by different users which gives them rights within the company.
The questions are based in a hierarchy format with currently 6 on the higer level, 3 on the 2nd level and 9 on the 3rd level. all the fields are going to grow in the amount of values assigned and the overall solution needs to allow for
the questions are assigned to each level with the first level sharing alot of the same questions and the 3rd level being most unique.

each user needs to be able to apply for many different paths of the hierarchy in the one application, or
there can be many application which the user can fill out, but due to the fact that most applications share a large quantity of data. application can be pre filled base on previously entered data.

currently i have a solution for this using multi select option sets but these are unsupported in the dynamics portal.