I thought I'd show you how I went about attaching mudguards to my rockhopper; which is not designed to take them.
Basically I used P clips and some different screws (that span the larger distance).
Apart from sourcing the P clips of the correct diameter and a selection of screws, it was easy as!
For the rear mudguard, I had to place the stays assymetrically so that I can still put my pannier on.
The attachments for the rear mudguard that I didn't photograph was simply drill work and cable ties