To have the link bars automatically calculated for you (and auto-regenerated if you reorganize the structure of your website) like your current FrontPage method does, you'd need to replicate the relationships among your pages in a database and do the link bar code generation with PHP. I've never heard of anybody doing that. My suggestion... don't bother. The goal should be a replacement method that is good enough, not one that perfectly duplicates the original. The end result will probably be an improvement, even if it doesn't seem so at first.
If you're lucky, you might find that one or more of the following applies to your site:
1) Some or most of the link bars are extraneous clutter that add little value to the page. If you remove them, not much will be lost. If you have a sitemap, you can just put a link to the sitemap on each page, and get rid of unnecessary FP-generated link bars. Every link bar you delete is one that you don't have to convert.
2) Many of your pages (or groups of them) have identical link bars. Those link bars can be replaced with static HTML link bar code. It is even possible to preserve the FrontPage-generated link bar code (which is already in each of your files), but just make it "non-webbot" code so that FP doesn't manage the code anymore.
Contrary to what you'd think from looking at FrontPage-generated link bar code, a basic link bar is very simple. It's just a set of hyperlinks grouped together:
Textual link bar (horizontal):
Graphical link bar (adding the <br> makes it vertical):
<a href="/index.htm"><img src="/images/home.gif" alt="Home" title="Home"></a>
<a href="/sitemap.htm"><img src="/images/sitemap.gif" alt="Sitemap" title="Sitemap"></a>
When I converted my site, I ended up just deleting most of my FP link bars even though I knew how to convert them. Instead, I made a top navigation link bar (Home, Sitemap, etc.), put it in a .php file, and included it into the header of each page.
There was one set of pages closely related to each other where I wanted to preserve the existing link bars from each page in the set to all the other related pages. After some study of the HTML code, I saw how to remove their status as "webbots" and yet preserve the HTML code that the webbots had generated. That left me with plain HTML link bars in those locations.
There are complicating details of doing the conversion that you'll need to understand by studying your code and using web searches if necessary to learn about how FrontPage works "under the hood". Go slowly and carefully.
The key concepts are:
1. You might not need as many links bars as you currently have.
2. The basic HTML code for a link bar is so simple that putting them into your pages manually is not that difficult, and the FrontPage method of managing the navigation structure, auto-calculating parent/child relationships, and auto-generating the link bar code might not really be saving you anywhere near the amount of time and trouble that you thought when you first started using the FP link bar methods.