unlock sidekick lx for sprint?

is it... possible?

and is there any way to use the web services/IM if you unlock it? (even for any other service?)

no not at all
one is GSM (t-mobile)
and the other is CDMA (sprint)

No it's impossible.
Because sidekick lx phone support GSM and Sprint is CDMA.
But you can use other GSM provider on your sidekick phone.If you are interested to use other GSM service on your phone,you will need to unlock your mobile.
You can unlock your phone by network unlock code.If you need an unlock code visit SuperUnlockCodes.com Here you can unlock your phone at low cost.