Jones is a Wales international with 104 caps to his name, while Johnson is a South African number eight.
Despite the club suffering from financial problems, chief executive Chris Thomas has insisted that it was important to add some fresh faces to the squad for the new season.
"While everyone is aware that the club is seeking investment as a matter of urgency at the moment, in terms of player recruitment and retention, like every other club at this time, we have been working hard to continue to put our squad together for next season," said Thomas.
"We are very pleased to have recently secured these excellent signings, whose quality will place us in a strong position for next year.
"But, while it is pleasing to announce further additions to the squad, of course our priority now is concluding a deal which will secure our financial future."
The pair will join Wasps in the summer.