Hello,
I have below problem.
My external solution generete a lot of pdf's with different templates with signature place (white font -> @signature@)

I need to use anato pen and make something like this:

Capture pdf.
Find "@signature@"
Insert Anato Pattern and print
Customer use Anato Pen
Insert signature to PDF.

It is possible using planetpress design and workflow?

To find the text shouldn't be an issue (see Talk function region()), but to find out the coordinates where it was found could be tricky, maybe doing a series of region() calls on subsequent lines, and when it's found you know the place.