Are your .wtv files set to open with WMC by default? The right click option to convert will only appear on a .wtv file that is set to open with WMC by default.
If the answer is yes then you may have deleted the regkey for the link. use this method to get it back.
1. run regedit.
2. navigate to " HKEY_CLASSES_ROOT\*\shell
3. right click on shell and create a New Key. call it Convert to .dvr-ms
4. right click on Convert to .dvr-ms and create a New Key. call it command
5.click command, in the right pane right click on the (Default) and click Modify
6. type the following "C:\Windows\ehome\WTVConverter.exe" "%1"