I have done things like this many times and I always went with the same approach.
You need to understand the XACML schema. Then you do a conceptual mapping to the BRFplus artifacts like decision tables, rulesets etc.
Then you write your XACML inbound interface (e.g. an RFC function module and/or a report for manual upload). Once the XACML is loaded you convert it to BRFplus by using the BRFplus API. Afterwards you save and activate everything. Aquestion you need to deal with is also if the XACML content shall be loaded into an existing application. Should it overwirte existing content or should it all be created again and again which may lead to problems long term. Further, in all cases I did I was able to identify a big deal of reuse. So I had created those reuse objects beforehand (data objects, rules, ...) and in the conversion XACML I had used the reuse objects whenever possible instead of generating same objects multiple time.
I know there are people that tried to do this by XSL transformations. My experience with this is negative as usually the complexity is too high for an XSL based approach.