How to include an external file in the theme, from outside the IPS installation?

[Vikestart]

How do you include an external file in the theme, from outside the IPS installation?

I was told that the path of the {insert="..."} tag is relative to the IPS installation, which makes it impossible to use it to include files outside of that.

Are there any other feasible ways?

The IPS suite is not the main CMS on my website right now. I use WordPress with ACF for most things as the IP.Pages app does not seem to be very capable of procuding complex repeating lists of data.

So far I've maintained all of the website's theme files in one place, which is in the WordPress theme folder, and I'd like to keep it that way, instead of having to store some of the files in the IPS installation....