Hi Marc! Almost all EF lenses, including the two you listed will work on the Canon Digital SLRs. On the Digital SLR they will have 1.5x the magnification that they would have on a 35mm camera. Your 28-90 will be like a 42-135mm and the 75-300 would be like a 112-450mm. This is because the image sensor on the Digital SLR is smaller than a 35mm negative.
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You haven't stated which body you have but the AF will not work with the extender if the effective lens aperture is too low (remember that a 2x extender doubles the focal length but halves the effective aperture).
Canon's own website states: "Autofocusing does not operate if the effective maximum aperture is
greater than f/5.6 (greater than f/8 on the EOS 1V and EOS 3)."
Canon EOS 500N has an EF lens mount, so all lenses with Canon EF mount will fit. All lenses with MF and EF-S won't fit. If you purchased Sigma 28-70mm f/2.8 EX, then it have EF mount and will fit EOS 500N.
There are 2 Canon fittings, EF for all EOS cameras and EF-S for 'crop' cameras. The Canon extender does not fit crop cameras. Whichever make you look at, make sure it will fit the type of mount you have.
If you have not yet bought your lens of choice, Sigma are good and do their own extender compatible with their own lenses but not Canon's. This may be a good option for you