It can be done ONLY IF AxCrypt can open the file itself. So if Axcrypt can’t open the file it can’t send through “Open in” to another app.
You can test yourself trying to open a Keepass db file. You’ll see Axcrypt not being capable to open it and no option to “Open in” something else. Normally other filemanager-like apps (i.e. DropBox, Documents by Readdle, iFiles etc.) can perfectly do that. More clear now?