Cannot import p7b certificate

Hello,
from an email-correspondent I got a p7b/PKCS#7-certificate for mail encoding, but I cannot import it. eM Client only asks for PKCS#12 with password. So is there a way to transfer the p7b certificate into a format that eM Client will import, or are you working on a future possibility to import p7b?
Thanks for your trouble
Tomedius