Thank you very much mapi. I had been looking around like crazy to find how you define your own stiffness matrix in abaqus and your pointer to the uellinea.inp got me on the right way. It soo simple and obvious when you see it but I couldnt find it in the manual. I always ended up in substructure where you can print out the stiffness matrix but not write in your own.
I write the syntax here so other might find it also if they search.
(from the file uellinea.inp under 4.1.11 in verification manual)
*USER ELEMENT,NODES=2,TYPE=U1001,LINEAR,UNSYMM
1,2,3
*MATRIX,TYPE=STIFFNESS
1.05E9, 0. , 0. ,-1.05E9
0. , 0.
**
0. , 0. , 0. , 0.
0. , 0.
**
0. , , 0.0 , 0.
0. , 0.
**
-1.05E9, 0. , 0. , 1.05E9
0. , 0.
**
0. , 0. , 0. , 0.
0. , 0.
**
0. , 0. , 0. , 0.
0. , 0.0