What do you plan to do with this converted DOS program? If you just plan to run it and nothing else, then you can use "System Exec.vi" in LabVIEW to run your DOS program.
If your DOS program is written in C, you might be able to use the LabVIEW function, "Code Interface Node" to pass data between your DOS program and LabVIEW.
Shan Pin Koh