Yes. Talk to your local Sigma lens parts department and ask for a Canon EF bayonet mounting ring, a Canon EF electrical contact housing (lens connector) and a black plastic rear shroud. Soldering required for modifying older Sigma SA mount lenses. It is not difficult and anyone with basic soldering skills will be able to perform this modification.
Yes. You may order a Sigma SA bayonet mount ring and a Sigma SA electrical contact housing from your local Sigma parts department.
If your Canon EF mount lens is Sigma brand to begin with, it is going to be pretty easy because the screw pattern of the mounts match exactly. If your Canon EF mount lens is of some other brand, some drilling and/or soldering may be required. For example, please look at