Those piano roll editors provide a convenient way to sequence MIDI note data. There is not, to my knowledge, any way in the MIDI standard to specify portamento between two specific notes. However, if you're only sequencing for one voice, you can send MIDI pitch bend information, like you have suggested, but this is global to the channel so you can't, say, bend one note while keeping the other at its original pitch (at least, not without duplicating your instrument on another track).
Given that, there are two ways I know of to attack the problem of portamento:
Just use pitchbend
Most instruments I've used that accept MIDI respond to pitchbend information, and so you can program this in your sequencer. You likely won't find it in the piano roll however, you'll have to find it wherever your sequencer stores envelope or automation information.
For Reaper, this is the view at the bottom of the piano roll that by default shows your note velocities. Click on the 'Velocity' chooser and you'll see that you can choose 'Pitch.'
For Ableton Live, this is another overlay in the Clip View (where the piano roll is). Click on the little "E" in the lower-left corner, which will open up your envelope editor. Choose 'MIDI Control' from the first chooser and 'Pitch Bend' from the second one.
Use a clever instrument
Many soundbanks and synthesizers (monosynths, in particular) have an automatic portamento feature. Generally there is a knob, usually labeled 'portamento' or 'portamento time,' which sets the amount of time it takes to bend the pitch to the next note, if that note is triggered before the previous note ends. Again, this only works in the context of single-voiced instruments, since with multiple voices it isn't clear if it should bend the note or play the second voice.
I personally find this to be a useful solution, since I work with largely electronic music and I prefer extending and reducing note lengths to editing envelopes, as I find it more intuitive. Your mileage may vary.
I am sure that many devices and plugins have implemented their own solutions, such as proprietary uses of certain MIDI CCs for controlling portamento behavior, but I don't know of any off the top of my head.
Edit: turns out there is a standard set of CCs for portamento control, see Henrique's answer. But it's up to the specific instrument to implement/interpret that, and like pitchbend I believe it affects the whole channel rather than targeting specific notes)