I would not recommend it. Hellknight signifier can be good, but commander just doesn't synergise well.
Cleric to hellknight has tons of roleplay value, but I don't think dipping for another domain is worth it mechanically.
First, you need 3 levels to get the first discipline, second your hellknight levels are treated as cleric levels for what abilities you access in that domain. That doesn't mean your cleric levels stack with it since they're not powering that domain.
If you don't care about advancing your cleric casting any further, I do think you could make a very flavorful martial build doing this, though.
Okay i can see that the commander probably isn't the best option. Would the hellknight signifier make a good combat cleric? Cause the bab seems like it would be a little low...
The BAB would be the same as any other cleric (3/4), but you get a bunch of abilities extra, all your casting, AND domains, inquisitions & mysteries still scale. A cleric only looses chanelling, which was never a huge thing to begin with.