Modifying this context to allow italics/bold etc. is possible by modifying the DTD/Schema. In the case of DITA this is called "specialization". However, there are some limits to specialization and this may be one of them. It is uncommon to allow bold/italics in titles.
If you do end up allowing bold/italics in titles you will also want to make sure your output stylesheets are configured for that as well. Eg. extracted titles are sometimes used for running header/footer or in a ToC or possibly Index. In those cases you often want to strip the bold/italics/etc.
