Merges between an input spooled file and a design (template) is usually done via the Interform workflow.
It is possible also to call Interform commands from your own programs if you want to decide when this merge should be done. Most customers are using the workflow to do the merges.
In the workflow, you are able to monitor an output queue, folder, webservice, MQ, DB etc. For each e.g. output queue you can set up rules for what to do (e.g. print, email, or create PDF files) depending on a lot of spooled file attributes.