I recently downloaded the Picross application by warner bros and had a look inside. The software uses downloadable .pxml files that you can open in notepad like so:
After lookind at these files I realised they could be edited easily.
0 represents a blank square while 1,2,3,4,5 and so on represent colours.
I tested my theory out by editing the neighbourhood pack (I have not yet managed to create a new pack)
As long as the .pxml file has the same name you may change the name of the pack.
Neighbourhood to "peter's custom pack" as seen here:
Success. When loaded into the App via diskaid the name changes!

Peters custom pack appears.
I took it to the next step by replacing the first puzzle "Dog" to "smile"
I edit the image using these numbers.
This represents the first row and its 5 squares:
<row>0, 4, 0, 4, 0</row>
0=none
4=red
This gives us two red dots.
Next row the same thing
<row>0, 4, 0, 4, 0</row>
Until we plan out the whole picture
<row>0, 4, 0, 4, 0</row>
<row>0, 4, 0, 4, 0</row>
<row>0, 0, 0, 0, 0</row>
<row>4, 0, 0, 0, 4</row>
<row>0, 4, 4, 4, 0</row>
The 4's represent the filled in squares making a smile. I uploaded my edited .pxml
Low and behold:
A whole new Picross!
You can even add new puzzles.
The problem is that this process is very time consuming. My coding skills are next to nought. I do think
that it would be possible to make a GUI program to generate <row> </row> files and to make them different colours. Maybe even inject the new picross into packs. I will need help with this. Post your thoughts or message me. Thanks
If in wrong forum please move.