yes you can. Just get as close to her as possible, by giving her gifts and being nice and
sh!t, then you can "screw" her.

You can also have a love triangle with the witch and the rogue. But eventually the rogue
won't want to share you and will make you choose. But you can deny it the first time and
buy yourself a few more "at-bats" with the girls.

FYI, as a female character you can also hook up with the rogue since she's bi.....that's
right you freak lol.

On the other hand the elf assassin that you can recruit is also bi.

You can sleep with her although it can be difficult, a sh!tload of things need to happen
you could have 100 in disposition towards her and still won't be able to sleep with her
using her dialogue options.

I think it's actually easier to sleep with her if you haven't slept with morrigan yet,
although even if you have it won't be impossible.

You need high enough disposition with her so that when you enter camp you'll have cut
scenes with her, such as her singing an orlesian ballad. Once that scene occurs you should
now be in favorable position to have the option of sleeping with her.

There is also another way that you can sleep with her more easily. First, as usual, you
need to raise her disposition very high, which is easily done with gifts and talking to
her and for maker's sake don't destroy andraste's ashes in front of her....

Eventually you'll be able to ask her about her past, tell her how you've "heard that bards
are assassins" she'll deny it at first but next time you enter camp she'll reveal the
truth. Then an extra random encouter should happen where orlesian assassins where sent to
kill lelliana. Find marjolaine and KILL her. Next time you go to camp lelliana will say
how she enjoys killing MAKE SURE you tell either "don't punish yourself for doing
something you enjoy" or "this is who you are" you have to answer one of these two in order
to "Harden" her personality. By then you two should have declared "feelings" for
eachother.

Once she's been "hardened" go to the pearl in denerim with lelliana in your party, speak
with Isabella, do not play her game instead persuade her to go somewhere more "private"
lelliana will then offer to join the two of you and you will have a threesome with
isabella and lelliana. You don't see any "sex" cutscenes but i guess it still counts.
After that, at camp, you should always have the option of "go to bed" with lelliana, which
can be interpretated as sleeping with her.

At first, she's a tough *** to crack, but once you sleep with her once you should always
be able to, unless, of course you choose morrigan over her.