Sure, you can add it.
A couple of issues to consider - if your materials are of significantly different size, the larger ones will stay on top.
Most people who mix fourite with sand/gravel suggest using at least 50% flourite.
Think of how the colors will look together.
I personally have no experience with sand in planted tanks.
I understand a couple of concerns are that it can compact - impinging root development, and is biologically inert - limiting availability of nutrients to roots.
I think folk have better luck with larger size sand.
And make sure it isn't adding anything undesireable to your water.