Solved!
1) Create new solution
2) Add entity which have the attachments subgrid
3) Export the entity
4) Extract files and open the customizations.xml
5) Find the control id="attachmentsGrid"`and copy the control
6) Do step 1-4 but with the entity which is missing the attachment grid
7) Paste the control at a relevant place (depens on where you want it)
8) Zip the files and import