add to speed without behaviour

So I have a sprite moving X-30*dt. (and other speeds as well)

Is it possible to add to this "speed" relative to its current speed, or will I have to assign a movement behaviour to do so?

not sure what you are doing but how about having an instant variable on the spite (called ivSpeed ?) and doing something like

X-(ivSpeed*dt)   (I know the brackets are not doing anything but it is easier to read in my mind)

then you can alter ivSpeed value to alter the sprites speed ?

See delta-time and framerate independence.

As RamPackWobble says, you can use a variable instead of a fixed speed like 30, but you must also use dt when modifying the speed for it to be framerate independent.

Thats what I need.. thanks! :)