Switches of this (and similar) kind can be configured and manually converted from latching to momentary, or radio-button mode (pressing one key releases others) by manipulating their mechanical elements:
(Attachment Link)
When the OP states
'to convert momentary to latching, latching to momentary, both to only-on-as-long-as-you-press-it'
his idea of 'momentary' which is not congruent with standard usage, seems to be similar to an 'Einschaltwischer'. Can those switches be converted into that? I don't see how.
To be really free in creating most of the switch actions defined in the related standards, one needs a different type of switch.
I have done that on several occasions, but it needs a combination of electromechanical knowledge, extreme dexterity and some not-so-common fine tools.
Then you can start with switches, that have open contact sets and you can convert them into any characteristic that you want. For example the functions 'Folgeschliesser' and 'Folgeoeffner' (sequential closing and opening contacts, which can apply to a set of three contacts making contact among all three or to two electrically separate NO contacts) can be build as well as wipers. Manipulating the actuation part will change between latching and momentary action.
Examples of such Switches would be the series FgBv87 of the former DBP, which was in use by all major telephone and telecommunications manufacturers. (See the larger switches in the Picture below)
Not only can their contact sets be manipulated, but there are very interesting switch variants by design. For example, I do have both a switch with a holding coil and several with a lever, onto which a tripping coil (or other mechanical actuation) can act. In the picture, both push-actuated switches and rotation-actuated switches are shown. The nice thing is, that they all share a mounting geometry.
Another type of switch which lends itself to such practices are Kellog switches, although their narrow definition of up-down or up-neutral-down lever actuation limits the possibilities somewhat. There exists even a series of miniature Kellog switches (the FgBv of which I sadly o not know) which share the contact set components with the FgBv87. The Kellog switches shown are momentary as well as bistable (or tristable ones.
One must sacrifice some specimen (or have some defective ones) initially in order to gain contact elements, the so-called 'pimpel' and the thin isolator sheets. An absolute necessity for such an undertaking is the availability of contact adjusting pliers and -levers.
Another possibility, which is seen on some more recent industrial examples of combination switch actions is the use of encapsulated snap-action switches in combination with a specially shaped actuator.