You generally need to bring an applicable skill to the table immediately on hire. With the exception of a few stable mid-sized companies that really love generalists and cultivating long-term employees, and fresh-grad hiring, most places can't take the time to ramp you up on a new skill AND their codebase before getting value back out of you.

But it also depends on the distance between skills (frameworks < platforms < languages), whether you know the industry or applicable business logic already, the breadth of your resume as a whole (a proven generalist vs a transitioning specialist).

But most importantly -- you can't really know from the outside -- don't be afraid to burden them with your resume. Worst case, they'll throw it out immediately and forget they ever saw it. Best case, they'll be in a pinch or spot a detail in your resume that you didn't even know they were looking for.