<p>Can you convert a 3D printer to a laser engraver?</p>
<p>If so is there a specific thing I have to buy?</p>

Yes, one can convert a 3D printer into a Laser Engraver, and it isn't even all that problematic.
You'd need to acquire a laser diode. These are usually not available with a lot of power - certainly not the hundreds of Watts that come from a proper CO2 laser - but depending on your printer, it might be pretty much a bolt-on addon.
The conversion isn't
too
invasive, and takes much less time than building a printer from a set.
All3D
converted an Ender-3 to have a laser within less than an hour. In their step-by-step guide, they wired it into the 5V part cooling fan port of their printer. Whatever laser diode you choose, the setup will be similar, as long as your diode works with 5 Volt power.
Once installed on the physical side, you will need to prepare your prints with a different "Slicer" that uses the commands for the fan speed as powers for the laser:
```
M106 S
```
sets the power to S, where S is a number from 0 to 255 ; Without an S value, it turns on to the last setting (if turned off via
```
M107
```
)
```
M107
```
turns off the laser.
One such slicer would be
Creality Workshop
, which would work with any Marlin-run printer, as long as you use the fan as the diode's power source.