You might be able to press fresh leaves between absorbent paper, and when they're dry, kind of decoupage them into place.

I've never heard of anyone using fresh leaves in PM.  Most leaves contain sugars and tannic acid, which I believe would tend to stain the rest of the PM, and the color of the leaves almost never stays the same.

When it comes to including botanicals into PM or papermaking, all I've seen done is adding dried flower petals to the finish layer. And most of them will fade when exposed to sunlight.

The only way I have preserved leaves is by soaking them in glycerin for a couple of weeks.  And that only worked on the heavier leaves that had already turned color in fall, like sycamore and oak leaves.  But I've never tried incorporating them into PM.

Of course, you could try it and possibly come up with something new, but......... I'm suspecting it won't work.

Sue

You could try one of the more translucent rice papers (really hibiscus, but everyone calls it rice paper).  Or one of the sturdier tracing papers that you find in tablets or on rolls.  If you're not in the U.S., I'm not certain what this kind of paper is called.  It's a rather "hard" translucent paper... it might also be called vellum, but a cheaper variety of it.  Parchment?  But not the baking/cooking kind, as that usually won't absorb moisture.

And try using methyl cellulose as your adhesive, thinned to the consistency of egg whites.  You may be able to find it in the form of cellulose wallpaper paste (dry, not premixed). But it can be hard to find. Paint and wallpaper shops may be willing to order it for you if they don't have it in stock.

And I'm not sure what will happen if you try to mix leaves preserved with glycerin and PM.  Let us know, right?

Sue