Not directly u cannot! I wanted too myself..cannot! 

Got methods but very troublesome...best u check with Maxis!

If you are the account holder of the Maxis (originating) lines
Bring your IC, go to a Maxis centre and request to change your supplementary line to a Maxis prepaid number. Maxis will open up a new prepaid account which contains one line only -- that suppl. phone number. Next, go to DiGi centre and request to MNP that Maxis prepaid number to a DiGi postpaid account , and sign up for DG Smart RM58.

Extra notes : MNP works on account basis , not line basis. So, any lines under the origin account of your existing telco will be moved to the destination account of the new telco. So if you want to move just one line, you'll have to do the above workaround. Reason being choosing to open a new prepaid account is simply because it's the easiest to create and easiest to ditch. You do not even need to topup any amount. Afterall it just serves as a temporary bridging account until you get to DiGi. This postpaid - prepaid switching should usually be FREE.
If you are NOT the account holder of the Maxis (originating) lines
Steps are similar to above, except to note that, if you are not the account holder and just the suppl. line user , and you (not the Maxis principle) wish to own your new DiGi account instead of the the Maxis principle, you will have to make sure :
Get your Maxis principle to write and sign a letter of authorisation to indicate that you as the suppl. line holder wishes to detach from the original Maxis postpaid account, and to open a new prepaid account with that existing suppl. number. A photocopy of the principle's I.C. is required to be presented to Maxis as well.