I took a look at example k2 plugin that I found on this site.
I saw that in xml file J1.5 standard <params> are used instead of J2.5 <fields> with clear comment on that:

...These parameters are parsed by K2 only and we decided to just make things easier for developers, instead of opting for double parameters in 'params' and 'fields' blocks.

I just wanted to check if that means that K2 plugin parameters are limited to J1.5 standard parameters?

The thing is that I need some sort of multiselect option (checkboxes, multiselect dropdown...) inside custom plugin and J1.5 standard parameters don't have this option

Is this going to change in 3.x series of K2?

J2.5 standard field types offer really wider set of different field types.

Lefteris, any comment on that?

Sorry for bumping.

Hi. You can still define custom elements and make a param what exactly you want. For example using in your XML:

will add this path to the parameters path. Then in this folder you can define whatever parameter you like. You can see several example of custom elements under the directory /administrator/components/com_k2/elements

Sorry for pubmbing, but I want to know where to add this code in the XML file: