Hi Jesus Valero,
What you Basis team said its not possible to connect 7.0 & 7.4 for Transport................
No that's not right its directly possible to transport between 7.0 and 7.40 see the SAP Note:
"1090842 - Composite SAP Note: Cross-release transports"
Happily Migrate to 7.40 and enjoy its very good feature's and also it will keep your system ready for HAAN migration 
.
Thanks & Regards
A.Dinesh