I want to add field object to an existing repeating frame.But when I drag the field, new frame is generated.
See Image below.

I want to do is put the field F_PO_NUMBER under R_INVOICE frame.
I'd suggest you to
- create a backup copy of that RDF file - you might need to restore the report if something goes wrong
- go to paper layout editor
- turn flex OFF (so that frames wouldn't stretch)
- select any field in the R_INVOICE frame (for example, F_INVOICE_NUMBER)
- copy/paste it (Ctrl+C / Ctrl+V)
- a new item (named F_INVOICE_NUMBER1) will be created slightly right & below from the original item)
- move it to a new position
- if necessary, switch flex ON (and, possibly, confine ON/OFF), but be very careful because you might spoil current layout, and - if you do that - it might be difficult to revert changes. Undo works only 1 level back
- change its name & source so that they are appropriate (i.e. F_PO_NUMBER)
- save & run the report. Hopefully, it'll be OK.
P.S. As you already have the R_1 frame (which is useless), I'd suggest you to remove (in paper layout editor) it, as well as current F_PO_NUMBER field (visible on your screenshot).