The very short answer to this is, “Yes”. However, it is not something that is built into the plugin (for a number of reasons included below).
The (freely available) plugin does not have a mechanism to do this “out-of-the-box”. However, it’s not an impossible task, and isn’t something that is unique to the plugin. There are many tutorials on the internet that describe how to create a WordPress login modal or JavaScript-driven form directly in your menu. Any of these that are compatible with WordPress are generally going to be compatible with WP-Members. WP-Members uses core WP functions to recognize and maintain login state. If WordPress sees a user as logged in, so will WP-Members. If a login tutorial you found on the internet works with WordPress in general, it will likely work with WP-Members, too.
It’s not something included in the plugin for a number of reasons. First is what is mentioned above – there’s nothing about the plugin that would prevent you from doing this on your own. Login is not something unique or proprietary to the plugin.
Second is a matter of taste. If I develop and produce a built-in process for the plugin to do this, it’s not going to fit into your theme generically. I’m still going to get the same question but with a twist – “How do I customize the login look-and-feel so it fits my theme”? If you just implement something custom from the beginning, it will look “built-in” the way you want it. Ultimately, you’re going to want to do it that way anyway, so doing it in the plugin isn’t productive use of development time.
Lastly, it’s a free plugin – the core is, anyway. For premium support subscribers, there are already some tutorials on this site that describe how to implement a login modal, and you’ll have support to tweak the design. There is also a modal script in development for the Advanced Options extension, which you’ll have the ability to customize via custom CSS.