It's rather easy, just some simple vector math.
If you want to increase velocity magnitude (speed) by a relative amount (a percentage) in the current direction, you can just multiply it by a scalar value, i.e (you have to include xs.inc):
PHP Code:
new Float: fVelocity[3];
pev(id, pev_velocity, fVelocity);
xs_vec_mul_scalar(fVelocity, 1.5, fVelocity); // 50% speed boost
set_pev(id, pev_velocity, fVelocity);
If you want to increase velocity magnitude by an absolute amount, you have to get a unit vector in the current direction, multiply it by your desired amount, and add it to the current velocity:
PHP Code:
new Float: fVelocity[3], Float: fDirection[3];
pev(id, pev_velocity, fVelocity);
xs_vec_normalize(fVelocity, fDirection); // Get unit vector that has the same direction as velocity
xs_vec_mul_scalar(fDirection, 100.0, fDirection); // Multiply it by 100, giving us a vector with magnitude of 100
xs_vec_add(fVelocity, fDirection, fVelocity); // Add these 2 together
set_pev(id, pev_velocity, fVelocity);