Like you I fancied doing this, knowing that certain Partners also had them fitted, but mine didn't.
A while spent on Servicebox showed that the armrest was available, plus the fitting bracket and screws necessary. I tried Ebay (being too tight to pay Peugeot prices) and found a brand new one from a 307. Checked Servicebox and the part number was the same apart from the last two characters to denote the trim. Bid on it and won it, then went to Peugeot and ordered the bracket and screws.
Today I tried to fit it.............and failed!
There is nowhere on my seat frame to attach the bracket. From the base where it joins the squab it is simply a tubular frame with no attachment points or brackets of any description. The fitting supplied by Pug shows four screws to attach it, but there are no holes on the fitting either. The diagram on Servicebox is useful to identify the part but not much good to help with fitting.
I can only assume that those that have armrests have a different seat frame from those that don't. If you get your hand up the side of the frame underneath all the padding etc, you might be able to feel some sort of attachment point, if you can't then I would suggest that you won't be able to retro-fit.
In the meantime, I have a new armrest and fitting that I can't do very much with until I figure out a way to get it to attach
P.S. The part no. for the armrest for the 307 is the same as the Partner and no doubt a few other Pugs too