Hi-
DBXL is going to act as a "central repository" for your XML data. So what does that mean? It means that if a user submits a new form (XML), DBXL receives it and maps it. DBXL can map data to SharePoint and/or to SQL. That data can be edited and resubmitted, and DBXL will map that data to keep SQL and SharePoint updated with the latest data.
In your scenario, SQL seems to be the (current) central repository. The problem with that is: what happens if changes are made to the SQL data and DBXL doesn't "know" about those changes? The XML stored by DBXL would now have obsolete data. Therefore, the InfoPath form would need to be carefully designed to always query SQL to obtain the latest data. That's not impossible, and Qdabra has put together solutions that can achieve this, but it's usually done as part of consulting work, i.e. we don't have a step by step guide on accomplishing this on your own.
Ideally, what I would recommend is this:
1. Design an InfoPath form
2. Create the DBXL document type (with SharePoint and SQL mapping)
3. Test it to make sure everything works as expected.
At this point, we know the solution works: the form submits to DBXL (from the browser, given your SharePoint mapping) and DBXL maps the data to SQL and SharePoint.
4. Then import the existing data into DBXL (using the Excel to XML tool).
This step means that we no longer have the scenario where SQL is the "main repository" of the data. DBXL now has all the data, and is mapping it as needed. If users need to edit and resubmit the data, that's easy, since the XML is stored by DBXL and can be reopened, modified and resubmitted.
There are free DBXL training labs available here, and videos available on Qdabra.com. DBXL offers a free trial, so you would be able to test out steps 1-3 above without spending any money and decide whether you wish t pursue this solution.
Good luck!