Oh, you mean the lock time.
Yeah, sure that should be easy to add.
FAIK, it's just setting the LockTime to a value other than zero.
You can do it yourself, if you like playing with source code.
Look inside
wallet/signtx.go, function
make_signed_tx() and change:
tx.Lock_time = 0
... to whatever you need. (like a value read from a command line).
I really encourage people to play with the code - if you are a programmer, especially if you did a lot of C, Go is a really cool language to code in.
I've been coding C all my life, but writing in Go is just so much more pleasant. And it compiles so quickly that at the beginning you cannot even believe it

Of course, there is a testnet switch -t and make sure to use it while you are having fun, to not be sorry at the end.
EDIT:
As for the lock time, I
read that in order for it to work, you would also need to set this:
tin.Sequence = 0xffffffff
... to a different value.
Thanks Piotr_n. That's very helpful!