I have some dark (walnut-colored) stained doors that I need to apply
some woodfill patches to.

- What is the best way to stain the woodfill?

In the past I have found that hardened woodfill (I use the Elmer's
stuff) does not take stain very will.

-Would it be better to mix a few drops of stain to the woodfill
*before* applying it?

Any other suggestions?