As Goldorak says, re-depositing the miles will not extend their original life span.
If the miles were transferred from Citi (and assuming you had to XP-earning flights since then), they will be valid for two years from the date of transfer. So, if you did the transfer in March 18, they will be valid until March 20 and, therefore, if you take a DL (or other XP-earning) flight before then, their validity will be extended for 2 years from the date of the flight.

What I am unclear about, and others might wish to clarify, is the following scenario:
1) I book a reward flight using miles that were going to expire. Let us say, for the sake of argument, the miles are due to expire in April 2019 and, during March 2019, I book a reward flight for January 2020.
2) I then take an XP-earning flight after the original expiry date of the miles but before the date of the reward flight (let us say: July 2019).
3) I then cancel the reward flight (say, in August 2019).

In that situation, do the miles re-credit to the account or do they get wiped out immediately on redeposit? In other words, does the XP-earning flight after the original deadline for use of the miles prolong the life of the miles that were on life-support through their use for a reward flight or not?