Creating advanced shapes

[Igor Grzegorski <bober emag pl>]

Hi,
I use Dia for designing flow diagrams and schematics in coal processing.
There are many standard "shapes" in coal processing, that can be used 
for all industry purproses, such as belt transporters, bucket 
transporters, basins, silos, doors, different kinds of valves etc. But 
if I create some shape, I can't modify it. For example belt transporter. 
It's always drawn as two circles connected with two lines. In standard, 
All circles in diagram have the same dimension, but lenght of lines is 
variable. And upper line has more or less "sticky points" where arrows 
can point. Unfortunately, if I create shape of that transporter, place 
few ones in diagram and try to have one of them longer then I can only 
strech it and have "eggs" instead of circles.
I haven't found any info about Dia's XML.
Is it possible to create shape that is (partially) modifable? Eg. having 
circles "blocked", but lines sticked to these circles and sizeable?
I attach example diagram with "shapes" created as groups that I 
"ungroup, modify and group again" when I need different transporters. 
One is angled, one is long and one has maany stick points where I can 
load materials.
The second shape "silo" illustrates another problem. Ends of lines can't 
be sticky. Because of that I can't create shape with "sharp tip", but I 
create tip made of veeery short line that has sticky point in it's 
center. Of course, all plotters draw it identically because of whole 
object's size, but it's very trycky.
Is there any way to get ends of lines "sticky" with others? Its possible 
to use poly-lines in many places, but not all are such trivial as the 
silo that I send in attachment.

Since i have made few own "library diagrams" from where I copy "shapes" 
to my work diagrams, I could make library full of industrial shapes 
based on my works - pneumatics, hydraulics, transporting of dry-goods 
etc. But I don't want to create unusable craps. If I could make good, 
universal shapes, I can willingly work on them and create all as shapes 
with descriptions.

What tricks are possible with Dia to do it?

Igor