On the Velleman support forum are some descriptions on how to install a heated bed.
Please note, though, that a heated bed by itself is not sufficient to print ABS - it will certainly go better, than on the standard unheated bed & printing relatively small components could work. But ABS really has a problem when you are reaching the higher builds, as those area’s of the print will remain unheated. To print ABS in the larger build volumes, you will need a heated chamber that can reach around 90°C. The Vertex K8400 can’t do that - not even if you enclose the build chamber, as the stepper motors (inside the build chamber) are limited to (I think) 60°C.
With that in mind…
I have purchased a heated bed & have the manual here - everything is ready to build it into the printer. But in the end, I decided against it:
- ABS is environmentally unfriendly and the material is not really evolving anymore.
- PLA plastics have significantly evolved recently & offer (in most cases) quite suitable replacements for ABS.
- PLA plastics are bio-degradable.
- PLA prints perfectly on the standard unheated buildtak bed - I now use the Buildtak Flexplate solution on the K8400.
Could I recommend, that you consider why exactly you’d want to print ABS? You might find, that the qualities of ABS have been superseded by recent developments in PLA and other high-end filament materials.
In that light, I’d also want to recommend to follow Thomas Sanladerer on YouTube, especially his Filaween series - where he is testing and publishing on many types of filament. His data will probably help you to find a PLA type filament with the properties you need.
It is certainly possible, to build a heated bed into the Vertex. But technology has come close to obsoleting it…
To be fair, I have another printer here that does have a heated bed: it’s an Opiliones 2L. I print PLA parts on both of them & they both do very well… It’s ok to have a heated bed. But if you don’t have one, the prints come out just as good. 
Hope this helps.
