Is the only way to add a quicklaunch in LXQt by mouse?

That's correct; as of LXQt 0.13 there is no way to modify this within the LXQt Panel settings; it's mouse only.

Sorry, it should read like this:

[quicklaunch]

alignment=Left

apps\1\desktop=/usr/share/applications/pcmanfm-qt.desktop

apps\2\desktop=/usr/share/applications/firefox.desktop

apps\3\desktop=/usr/share/applications/qterminal.desktop 

apps\size=3 

type=quicklaunch