My preferred method would be to introduce the telomerase in a method
such that the cell does not have control over its existence. For
example, add in a short artificial chromosome to the cells containing
the telomerase gene but a different kind of telomere which the telomerase
can't affect. Set up the artificial chromosome so that after a
specified number of divisions the shortening of its telomeres either
aborts its replication or, better, inactivates the telomerase.
That way you could protect the cells from senescence for a limited time
but after the desired number of replications the cell loses the telomerase
and returns to the normal replicative control and normal cancer risk.