So here is the thing. I've added all my clients via WHM and now just got WHMCS. So my question is, what's the best way to move my clients into WHMCS?

There's an import feature in WHMCS that allows you to import your clients from WHM to WHMCS.
Utilities >> WHM Import Script
Hope this helps.
Jamie Hann

As said above the feature within WHMCS for import will do this for you. If you have any problems with it contact Matt and his team at WHMCS.com :)
Good luck.

And also i think in their services at whmcs.com they have a paid service for doing this.
You could even think of that if you have problems.

you can go to sql base :) its not easy, but work for sure !!