hezz, uCoz provide a maximum of 10000MB of space per each website after you have paid for the increase. It is not possible to increase the sites disk-space after you have reached this limit. 10000MB of disk space however is a massive amount of usage already, if you don't mind me asking for what reason exactly do you want the extra disk-space?
As to utilizing other hosting companies it is possible. Cross-system usage is not that common though and can cause unnecessary load on the servers especially if you are sending a lot of requests to different hosts.
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I am willing to buy a package including a shopping cart and i think this module will contain data about products, catalogues , images and more and also orders ..etc and i am afraid it will need more space after years... That's it