You can always create a JMP data table from the R Square table you are referencing, and then save that table as a csv or txt or slsx file. to then be read into your VBA script.
I am not quite sure what you are attempting to ask. My thinking is that you have 2 lines that are attempting to serve the same purpose. The first line uses a JMP Join Platform to add a data table to an already existing data table. Is the second line attempting to replace values in the dt3 data table?