I'd think the light fixture wiring might be long enough to reach the romex using wire nuts - if you can get the wires exposed and free to use them.
Another option - there is a product made by Tyco that allows a romex splice without the use of a junction box. They are "usually" allowed by local code. I've never used one but it will prevent you having a junction box right there on the wall - as you can't hide a splice in a junction box within a wall per code. I assume with the white jacket that's 14AWG.
Here is a source:
https://www.zoro.com/te-connectivity-non-metallic-splice-14-to-12-awg-300v-wt-cpgi-1116377-2/i/G3673695/