If you look in the HRC file you will see:
A number for the bones,
then you see the bones in that structure:
PointB
PointA
Length of the bone
Number of parts and the name of the RSD file(s)
If you add a bone you must write it directly under it's parent point. As example:
D
A
...
has to be under
B
A
...
if you write it under
C
B
it wont work.
Let's assume you want to add a tail you need to add it to the root which should look as that
hip
root
3.6045
2 ACGE0 ACGE1
tail
root
3.6045
1 TAIL
Does that help? Don't forget to update the number of bones with the one you have added.