Would a space based magnifying lens (such as Hubble) be more efficient than a space based parabolic mirror to propel solar sails?
The answer is a clear no because Hubble Space Telescope (HST) uses a fixed focal length of 57.6 m. The only thing you'd achieve by re-purposing it as a solar concentrator would be to melt its focal plane assembly and likely everything around it. HST is a Cassegrain reflector and its mirror assembly is utterly unsuitable to applications where adjustable focal point is required because they're fixed optics. HST actually even required an in-orbit repair because it was out of focus due to one lens being out of position for 1.3 mm!
For what is worth, never point any reflector telescopes towards the Sun without specially made front optical filters (like e.g. H-Alpha filters). Use specially built refractor telescopes that don't use plastic parts that could easily melt, and only observe the Sun indirectly via a screen, say a so-called Sun funnel. They are optical concentrators, but their focal length is suitable for on-focal-plane observations of faint distant objects, not to redirect strong incident light to a distant location in a narrow beam. You can't make a laser-like assembly out of any telescopes. If you ever collimated a Newtonian / Dobsonian primary and secondary mirrors using a laser collimator, it would become pretty apparent why not. And Cassegrains use both hyperbolic and parabolic mirrors and refractors concave and convex lens assemblies. Their focal length is fairly limited because of this.