Josm prefilled preset

You can add multiple key & value pairs in the XML file of the preset.

I added key1 & key2 as given below in the "Crafts" preset downloaded from https://josm.openstreetmap.de/josmfile?page=Presets/Crafts&preset

<item name="beekeeper" de.name="Imker" fr.name="Apiculteur">
    <key key="craft" value="beekeeper" />
    <key key="key1" value="value1" />
    <key key="key2" value="value2" />
</item>

Selected a POI, pressed F3 and selected beekeeper and all the 3 key value pair were added to the POI.

Alternatively you can copy the tags from a text file and paste them into a POI. You need to have the tags and their values something like this:

craft=beekeeper
key1=value1
key2=value2

Copy these text and paste it into the POI using Ctrl-Shift-V.
Hope this helps.

Yes it's possible to add presets to your toolbar, but I'm not sure if they are expended by all previous values etc:
http://wiki.openstreetmap.org/wiki/JOSM/Advanced_editing#Tagging_Presets

You might also use F3 key, as the list is ordered by your editing history, so most recent preset comes first.
Also sometimes it's useful to add an object with all tags and then just using D key to dublicate the object. So you need just to adapt very few tags.