I hope this explanation about DEM Files Manipulation and Export may help you:
DEM Files Manipulation and Export
Visitor 3 can't import of DEM files but you can easily convert DEM files to XYZ files using 3rd party software. First choice for this task is MicroDEM. How to export DEM to XYZ from Microdem 6.03:
Start Quickview application from Microdem folder.
Select File -> Data Manipulation to open Data Manipulation window (or alternatively click at In<->OutIcon in the Microdem's toolbar). Note that you must select this mode before you do any other actions in Microdem, otherwise it's disabled.
In Data Manipulation window select Export -> ASCII XYZ, full DEM, open DEM file you want to export and select XYZ file. You're promted to ask coordinates units, use UTM for XY and meters for Z.
Tips and tricks: Another good and freely available DEM utility is 3dem. It's more user friendly and faster than MicroDEM in many operations. The fastest and most compatible file format to use to transfer files from 3dem to Microdem is GeoTiff. Here are some of tips and tricks how to process DEM data using 3dem and Microdem to prepare them for Visitor:
Patch missing DEM data to avoid holes in the map: Operation -> Patch missing data in 3dem; Edit -> Fill DEM holes in Microdem Data Manipulation.
Resample DEM to desired size and resolution: In Microdem Data Manipulation Resample - DEM, change X, Y spacing from current value to desired value
Merge DEMs: In 3dem just open multiple DEM files at the same time to merge them; in Microdem Datamanipulation select Merge -> DEMs, consequently open DEM files you want to merge, press cancel in Open merging file selector after last DEM is added and fill the target file name in merged DEM file selector.
Crop DEM: In 3dem, use Operation -> Select Smaller Area, select the area and press Enter; in Microdem Data Manipulation use Subset -> DEM rectangle and select the method you prefer.