Hello, Enrik.
Nimble can use only streaming networking protocols as input. (e.g. RTMP, SRT, MPEGTS, etc Please check Transport section of the following documentation page for the full list: https://wmspanel.com/nimble/)
If you can convert Decklink input to any of the mentioned above it will work.
You may try FFmpeg to republish streams to Nimble, as it seems to have Decklink support: https://www.ffmpeg.org/ffmpeg-devices.html#decklink
Unfortunately, we have not hands-on experience with Decklink cards, so cannot provide more detailed information.
Thank you.