A few things I can think of:
You can enter simple formula’s in KiCad’s entry box, for example if you enter 4+5 then the resulting coordinate will be 9.
I’m not certain what the SVG output does, but it probably plots what it thinks is the defined paper size, and then KiCad’s (0,0) is the upper left corner of the drawing area.
What you can do is draw your PCB as you do now, and when it’s (nearly) finished you can drag a box around the whole PCB to select it all and then use Move Exactly [Ctrl +M] from the popup menu under the right mouse button, and give it an offset of for example (100, 100) (Positive Y is downward) before plotting to .SVG.
If you later want to do a big revision, you could move the PCB itself back for that. It’s a workaround and a bit of a nuisance, but I do not know of a way to move the real origin relative to the paper.
Removed the quote & my remark. I made a mistake and read “can” instead of “can’t”.