Does eDocBuilder work well when we already have shells or masters printed?

e-DocBuilder works well for this situation. You can upload the artwork that is on the master as an image and set it to be for proof only and not included in production PDF. So, while designing the template the user will see how it looks completed, however the production PDF will only contain the variable information that the customer fills into the form.
To do this: You will create a field on your master for the graphic image proof that will represent the area that will be for proof only (the information that is on the shell/masters). In eDocBuilder Admin when you are setting the parameters for your template, you will upload this proof image to the Assets tab under "Graphic Files."  
Under the "Field Setup" tab you will you will set the Field Type as an Image from a Graphic File. In the Data Capture Tab select the Proof Graphic File you are using. Under the Format tab select "Proofing Only" under "Field Usage."