Are you trying to merge the DEM with the imagery as a fourth band? I don't think elevation is a valid dataset for land use classification. However, as you indicated, the pattern of the land derived from the DEM might be better. You may want to look into Composite Bands. That will add your land pattern file as a new band to your imagery. The composite bands is available to all license levels of ArcGIS.
The ESRI documentation states that the tool:
Creates a single raster dataset from multiple bands and can also
create a raster dataset using only a subset of bands.
Once you have that, you can use the Image Classification toolbar.
The Image Classification toolbar provides a user-friendly environment
for creating training samples and signature files for supervised
classification. It also serves as a centralized location for
performing both supervised classification and unsupervised
However, this is only available with the Spatial Analyst extension.