No, sorry, I was not clear. You can't turn directly a pdf into a fillable pdf.
Here is how I do it:
-I make a .png background using a graphic program (Photoshop, but GIMP would work also, of course, or any graphic program), at 150dpi and at the size I want the finale page to be (letter, A4,...). All the fields names should be visible on that background, because the next step, in Open Office, shall only place the fields to be filled.
If you want to go from an already existing document in pdf, you'll have to open it in your graphic program and save it in an image format (I use png, but any image format usable in Open Office should work).
-Then, I create a new document in Open Office, with all margins set to 0 and I import the .png image I have made as background (it should fill the whole document).
I define the fillable fields (I mean the places where you can input the values) and export the document as fillable PDF.
-That's it.
I have made a few of my own, you can have a look at the result here:
<a href="http://toybox-sw.blogspot.be/search/label/Character%20Sheet" rel="nofollow">http://toybox-sw.blogspot.be/search/label/Character%20Sheet</a>
It is quite easy and you can use only free programs if you want. The placement of the fillable fields can take some times (depending on how much there are).