Nitrokey3 and GPG-Keys with unusual length + Multiple GPG-Keys

I want to store a GnuPG-Key with an unusual bit size:
sec rsa3744/7C3192F14CF2D85A
the subkeys are rsa 2048.

when I try to import them into my nk3 (latest firmware version); i get a “bad secret” error. even when I try to use different keyysizes for the key-attrs in the admin-commands (I tried 2048, 4096, 3744), it didn’t work.
what do I need to do to import this key to the nk3?

And as far as I am aware, the nk3 should be able to hold three different GPG-Keys (including subkeys). how can I import a second and a third one?

best regards, Hanno