What is the correct conversion of dried whole vs. ground fennel?
If you're going to grind the fennel, you can just measure it after you grind it. Since you haven't mentioned grinding it, I'm going to assume you're planning on using whole seed. That being the case, I'd figure about 1 TBS plus 1 tsp of whole seed to equal 1 TBS ground.

EDIT: As Logophobe mentioned in comments, you're not going to get the same burst of flavor using the whole seeds instead of ground. If you have a spice grinder, use it. I use a cheap rotary coffee grinder that I picked up a thrift store, it never sees coffee, just spices. 

You could also use a mortar and pestle or even just slightly crush the seeds with the bottom of a skillet. Depending on the application, that could really be the best of both worlds. Lightly crushed seeds can usually substitute pretty close to 1:1 for ground, because they're going to have more flavor. Ground spices lose flavor quickly.