I have created a structural model in CAESAR II with user-defined section properties for multiple beam sizes (European sizes, not found in the CAESAR structural databases). The model returns accurate results; however, the shape of this custom beam is a solid rectangle. To avoid any confusion with the end customer, is there a way to make this custom section actually look like a wide flange beam?

No, because the software doesn't know the "user shape" is a wide flange. It could just as easily be a channel or a T-section.

when will developers insert european beams inside caesar? those beam are widely used in europe....

Some are in the German database.

The name may be however different. Example HEA is IPBI, HEB is IPB, UPN/UNP is U.

Thanks for the input and information regarding the European/German crossover designations!

Welcome
more info here

https://stahl.webstexx.de/

it is in german but now all the pages can be translated

do you mean this page?
https://stahl.webstexx.de/stahl-wiki/ipb-heb-traeger-ipb-heb-profil/

and this one?
https://stahl.webstexx.de/stahl-wiki/ipbl-hea-traeger-ipbl-hea-profil/

yes