Theoretically you can relate reflectance measurements to absorption measurements. This is frequently done on solid samples using a device called an integrating sphere. https://www.newport.com/t/integrating-sphere-fundamentals-and-applications
Your problem is that if the reflectance isn't truly diffuse, converting your reflectance to absorbance won't give you a representation of the bulk absorbance properties of your sample. Moreover the reflectance measurement usually needs to be calibrated against a material with a known standard reflectance, presumably to account for sensitivity variations in your instrument.
Could you do this with a handheld reflectance colorimeter? Maybe.... but it's hard to get good absorbance measurements from an integrating sphere, which is intended for this purposes. Because of that, I don't know that I'd trust any absorbance measurement made from a reflectance colorimeter, certainly not a quantitative measurement.
UV Spectrophotometers are cheap and pretty ubiquitous - can't you find one somewhere to use for your measurement?
ALSO: Please don't cross-post.