You can definitely do it,
Here is how I just tried on one of my Trail Instance.
I added new Tab as you need, I called it "HTML Page"
On this Tab I added Webresource, you can add Iframe as well and call your external WebPage.
For my simple use case I created a simple HTML page as webresource in CRM and configured it to Webresource tab as below

Sample code for HTML. Dont worry about long html file. Mostly it is bla bla. What is of our importance is <body onload="myFunction()">
and then in
<script>
function myFunction() {
debugger;
alert("Account Id when from fromcontext is ");
alert(parent.Xrm.getformContext().data.entity.getId());
}
</script>
complete HTML code below
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html>
<head>
<title>My first styled page</title>
</head>
<body onload="myFunction()">
<!-- Site navigation menu -->
<ul class="navbar">
<li><a href="index.html">Home page</a>
<li><a href="musings.html">Musings</a>
<li><a href="town.html">My town</a>
<li><a href="links.html">Links</a>
</ul>
<!-- Main content -->
<h1>My first styled page</h1>
<p>Welcome to my styled page!
<p>It lacks images, but at least it has style.
And it has links, even if they don't go
anywhere…
<p>There should be more here, but I don't know
what yet.
<!-- Sign and date the page, it's only polite! -->
<address>Made 5 April 2004<br>
by myself.</address>
<script>
function myFunction() {
debugger;
alert("Account Id when from fromcontext is ", parent.Xrm.getformContext().data.entity.getId());
}
</script>
</body>
</html>
Also on Form Load of account I added additional Javascript. This javascript will create global variable which could be called from your webresource.
Article Link for additional Javascript
Sample code used for Javascript below
formContext=null;
function onload(executionContext){
debugger;
var formContext = executionContext.getFormContext();
Xrm.getformContext = function (){
return formContext;
};
Xrm.getParentAttribute = function (attrName) {
debugger;
return formContext.getAttribute(attrName);
};
Xrm.getParentControl = function (attrName) {
debugger;
return formContext.getControl(attrName);
};
}
Final Result will be something like below

Summary:
- Create Wberesource/Iframe
- Create Additiona Js on Load
- Use global variable in your webresource.