I am trying to create a single document with numeric page numbers but will also an appendix with lower-case roman characters for page numbers
The numbering style is a property of the page style in use. You need two page styles, on the Page tab of the page style definition you can select which kind of numbering you want. Then, you need to change from one page style to the other through a page break: Insert → More Breaks → Manual Break → pick the style. In that last menu, you can also restart the page numbering.
In addition to the project documentation, you can check my free book about Writer for more information about the use of page styles and page numbering, among other topics.
You will find https://wiki.documentfoundation.org/Documentation/Publications#LibreOffice_Writer_Guide Chapter 5 gives lots of useful advice on numbering pages in documents and books.