Hi Fabien,
There isn't a way to do that. The field types are predefined. You can see them here, there isn't a type that allows you to refer to an object (other than User). The drop-down and multi-value are both for strings, there isn't any field type that will allow such a dynamic lookup:
The "Parent" as well as all other relationship between Agile Central's objects are predefined within the Object Model and not extensible or modifiable.
You can find more on fields and custom fields at: Create and Customize Fields | CA Agile Central Help
If you'd like to pursue this requirement then you'll need to post a request for a future enhancement. You post it in our Ideas site at: https://ideas.rallydev.com . If you have no username then request one by emailing to: ac-ideas@ca.com
Thanks,
Sagi