Reading this, I wonder what you are actually want to do? You talk about a standalone application in the first post but then mention a DAW in the second post which is obviously only needed if you want to run a plug-in. As JUCE can be used to build both, the answer depends a bit on what you want to build.
Generally, I don’t really know Xojo but a quick google search revealed that it is based on BASIC, right? I don’t think that there is any easy way to port such code to C++. If you are experienced you can of course integrate e.g. a JUCE processing core built as a static library in any kind of application written in any language that allows integration of C libraries, but I’m not sure if that is a good advice for you, this totally depends on your skill level and the complexity and structure of your application.
Regarding a WYSIWYG GUI editor: There is no officially recommended/supported solution to that, the current JUCE state-of-the-art approach is hard coded C++ GUI code. This gives you flexibility for highly custom GUIs but can be a lot of work for other use-cases. You can try to use the Projucer live build engine to achieve JIT compiling of your GUI code, although I stick to traditional recompilation as I don’t use the Projucer. There is a deprecated WYSIWYG GUI editor in the Projucer which has been deprecated and is disabled for new projects – I would not recommend using it. Furthermore there is no support for native widgets in JUCE.
Still there are community-driven solutions that add such functionality to JUCE through third-party modules, I saw e.g. @daniel’s Foleys Plugin GUI Magic being mentioned in this context here and there (although I never used it and I’m not sure if it’s suited for standalone applications), just search the forum a bit.