1. Convert the Vic game in Victoria.
2. Copy converted save game to DD\save games
3. Load up the converted Vic game in DD scenario menu.
I presume that you've already done these steps, and played long enough to find that you cannot research secret weapons.
4. Now use NOTEPAD.EXE to edit the game you've been playing in DD.
5. File to edit should be in DD\savegames folder, "whatyounamedit.eug"
6. Search (notepad >edit>find) for "sleepevent".
7. Near end of file you will see: sleepevent = { } event = "suchandsuch..."
8. This phrase pattern repeats and tells where to get the text file to load for each event group: event = "db\events\suchandsuch.txt"
9. Add in the missing secret weapon phrase as I described in the earlier post.
10. Save the file, with notepad. Exit notepad.
11. Start up HOI2/DD; load the file from scenario list.
This all assumes that you've already converted the Vic game and have been playing for some time in DD. For a new, fresh Vic conversion the process is the same except you only want to load and play the game for 1 game hour to allow DD to do its secondary conversion on the savegame file. So, you'd do steps 1-3, save the game through DD - almost immediately after loading it. Exit out of DD and do steps 4-11.
Note, there are a whole bunch of standard events, like ai events for each country that the Vic conversion will not have (like anschluss). If you want any or all of these you can copy them from a "normal" DD save game (using notepad) into your converted Vic save. But you have to do the editing of the converted Vic save file AFTER it has been loaded and saved in DD.