Dependent multiselect instead of dependent dynamic select

We run SuiteCRM 7.7.4. We want in our accounts-module a field ‘type relation’ with 5 choices (parent-dropdown):
[b]- Client

  • Ex-client
  • Prospect
  • Partner
  • Supplier[/b]

and then as child-dropdowns for the choices above as multiselect:
Client

  • option 1
  • option 2
  • option 3
  • option 4
  • option 5

Ex-client

  • option 1
  • option 2
  • option 3
  • option 4
  • option 5

Prospect

  • option 1
  • option 2
  • option 3
  • option 4
  • option 5

Partner

  • option 1
  • option 2
  • option 3
  • option 4
  • option 5

Supplier

  • option 1
  • option 2
  • option 3
  • option 4
  • option 5

It’s no problem with dynamic dropdown but we need it to be a multi-select.

Solution would be greatly appreciated.