The only tool that can get the face of a board flat is a jointer. As Louie said, you can make a sled, and use shims to run a board through a planer, but it will take 10x longer than running it through a planer.
You can also mount the boards to your cnc, being careful not to distort it, and flatten it that way. But a good jointer can flatten a board in seconds.
We make a lot of curved moulding, and I CNC cut the blanks. With are vacuum table, the boards must be absolutely flat to hold them down. To get them flat, they must be run on the jointer. If run through the planer only, they won't be flat enough.
After I saw
this, I had ideas about building a 12" jointer myself. But since I have daily access to a 16" jointer, I'm just going to sell my 6" and upgrade to an 8" with a helical head.
My recommendation, would be to keep the planer and use it as a planer, and look for a craiglist planer to get the head from to build a jointer. You can also find cutter heads on Ebay occasionally.
If you do plan to build a jointer, be aware that it's critical that the tables are absolutely flat.
And make sure that the cutterhead is mounted SECURELY.
And make it a habit of using push blocks.