Various graphical patterns can be typically created from both bitmap and vector sources and later applied to a new graphics file to fill out some elements with such a pattern.
Generally speaking, many graphics programs can create their own pattern through a dedicated function, typically found as some kind of pattern library. When you save the pattern made from BMP file, you essentially make bmp to pat conversion. Some graphics converters also offer this function, but it's better to create patterns for specific software in that particular software.