Change Caption during export with plugin

I would like to change caption during export with a plugin.

I've got a website with a galery made with piwigo and I use the piwigo's plugin to publish / export my photos to the website.

I'd like to complete my caption with something like that for example:

New Caption =>

{city} - {country} - {dateTime[%B %Y]}

{copyright}

{caption}

I've tried different solution without success:

1 - Using LR Transporter plugin

It's not the solution because this plugin change caption in LR. I want to change only during export.

2 - Using rc Exportant plugin

I don't manage to change something in the preset.

I've got a bad level in english and I don't really understand what to do to make what I want

3 - Doing my own plugin

I've tried with lightroom SDK.

I manage to do a box in export box to complete some metadata.

But I'm not a programmer, only a photographer, and I don't know how to do after.

I know that it's possible to do something like that. I use jf Flickr plugin and ther is a place to do that in this plugin (Metadata Management / Image Title - Description).

I'd like to do exactly the same with piwigo's publish.

Can someone help me to do that?