Answered Is it possible to login a user programmatically in frontend?

Technically ... this is possible (correct me if I'm wrong) ... the general idea is that a user being logged in boils down to them having a session.

Step one is to load the user object (I'm assuming in your case you already have a user in the DB) ... once the user is loaded into, let's say ... $user ... do this ...

if (!$user -> hasSessionContext('web')) {
    $user -> addSessionContext('web');
}

It just creates a session for that user at the moment ONLY if the user doesn't have a session.

Feel free to ask question ...

Update:

There's a gist on Github with a code snippet here; https://gist.github.com/itsmrwave/6586756

    $contexts = !empty($scriptProperties['authenticateContexts']) ? $scriptProperties['authenticateContexts'] : $modx->context->get('key');
    $contexts = explode(',',$contexts);
    foreach ($contexts as $ctx) {
        if (!$user->hasSessionContext($ctx)) {
            $user->addSessionContext($ctx);
        }
    }