I've written a script, a series of scripts really, to create caption frames. Here it is on the wiki:
https://wiki.scribus.net/canvas/Creating_Text_Frames_for_Image_Captions There are 5 scripts, the first four of which place the frame in a set position, bottom, top, right, or left. The last one is the previous 4 mashed together, allowing you to input the location. You can select one or more frames to operate on at a time, but only one location for each running of the scripts. Since I didn't error-check for image frame(s), you could actually create "captions" for any kind of frame. Greg
