You can mix it in during the rendering. For example:

http://www.openstreetmap.org/#map=10/37.6719/-122.3376&layer...

Being overly pedantic, there is more or less wide opposition to getting such data sets "into OpenStreetMap", it's better to treat it as a second data set at render time (as above).

People have been displaying contour lines and hill shading in OSM for years. There's no good reason to actually import the data itself into OSM,.