Smith and Lethbridge impressed for both the Academy and under-23s last season.
The former also scored a hat-trick at Moneyfields earlier this month and was on target in the win at Bognor.
Smith and Lethbridge reuniteÂ with former Pompey first-team coach Blake, who is now in a similar role at the Rocks.
And he insists the pair will give Bognor the physical presence and goalscoring threat they lacked last term.Â
Blake told bognor.co.uk: '˜It's absolutely huge for us.
'˜I know them and had a chat with Jack.
'˜You sawÂ with Lethbridge in the first half that he had three or four shots just off target.
'˜He's going to be a real help for us. We've only had Jimmy (Muitt) who has been an attacking player. Marley (Ridge) has had to play out on the right and Harvey (Whyte).
'˜Them two players coming in will make us so much stronger in that area and that frees Marley and Harvey up and we need that physicality.'