How to create new shapes in Dia?

[Balázs Tőgyi <togyibali gmail com>]

Hi!

This is Bali, from Hungary. I would like to create a diagram about a
heating system with some special item. Could you please explain what
is the process to create new shapes and integrate them into Dia? By
downloading and extracting an example.zip from the repository I
noticed that:
- xml is a list of the items
- png is the picture itself
- and there are .shape files with each .png files, does it needed, and
if yes how can I create them?
Where to put the files or how to add them in Dia?

I would be so glad if you could help. In case I can create a new
repository of heating system engineering I would sure share with you.

Best wishes!
Bali