You can probably just skim most of this, but I've written step by step instructions in the event someone else needs the additional info. Its also written based on GeoRAS 4.1.1 as each successive version seems geared towards pushing people into HEC's RAS mapper(or it just flat out doesnt work on our machines)
1)In HEC-RAS you can export the GIS data out of RAS by going to(in the main HEC-RAS window) File->Export GIS Data...
2)In the new window check the box for "User defined cross sections" in the lower left.
3)Back in HEC-GeoRAS, convert the exported ras file to XML by selecting the button with two arrows going in a circle(I'm using an older version of GeoRAS that still works with 9.3 so newer versions may be different).
4)In the RAS Mapping tab select "Layer Setup"
Type in a name in the box next to new analysis, select your XML file for RAS GIS export file, dont worry about the terrain section, then just select a folder you want the data stored in for Output directory.
5)In the Ras mapping tab, select "Read RAS GIS export file"
This should automatically create the following files: River2d(centerline), Bounding Polygon, XS Cut Lines, and XS Cut Lines 3d.
6) the XS cutlines 3d is a 3d polyline of whatever is in RAS. When you go to modify your TIN, set the elevation data source to be "feature Z value" and it should work.
I'm not sure if this will work quite how you want it though because it will only have the new data at those cross sections and it will just linearly interpolate between them when you update/create a tin from them.
Hope this helps. Let us know if it works for you or you find a better way.