By the looks of it they just fit over the strut then can a diferent top cap then you have a coilover strut.
About this product
Converts struts and shock absorbers into "adjustable ride height" platforms so that the desired ride height can be achieved using a "C" spanner. This universal kit is suitable for any 50 mm diameter front or rear strut and includes : 1 x seamless threaded tube, 1 x locking ring, 1 x spring platform, 1 x top cap(pilot holed to be drilled to size) per shock asorber.
You need to make sure these will fit. The Astra conversion kit uses a larger diameter threaded sleeve and 2 1/2" ID springs because the struts are too large for the 50mm sleeves. Standard top mounts are OK (at least they are on the Astra).
The top caps are quite thick and tend to reduce the amount of suspension travel by about 15mm compared with standard - this limits how low you can go unless you are prepared to be on the bump stops all the time.