Create various image sizes ad hoc

Im making my theme responsive and hence would like the images to have various sizes depending on device, resolution etc.

Normally i would just add make a add_image_size hook to image upload, however the theme im developing is already in use and the sizes needs to take effect for previous uploaded images aswel.

Is it possible to create the various image sizes ad-hoc? perhaps some hook i dont know about?

Best Regards, and thanks in advance