“for the hill country can be yours.” The hill country could give Ephraim and Manasseh more land if they would take it; God had given that land to them as part of their inheritance already. Joshua is not giving the tribe of Ephraim and Manasseh any more land than they had been allotted, but if they clear what they inherited they can possess it to the furthest border. The land, when cleared, “will be yours,” that is, you will be able to live there. The text would have been easier to understand if Joshua 17:17 and 18 had been divided up differently and the last part of 17 was in verse 18.
“For you can also drive out the Canaanites.” Another way that the tribes could have more land would be to drive out the Canaanites, which they were supposed to do anyway.