If you're looking for a way to do this without using Esri software to convert, I don't think it's possible. If you are open to using multiple steps, then it's more likely. The first issue is that lat/lon fields are not geometry. They're coordinates, probably a number field. We need to get them into a spatial format.
One way to do that is (like Xander Bakker said earlier) to use XY events and make the lat/lon pairs into a point layer. The problem with XY event layers is that they're static (Make XY Event Layer—Help | ArcGIS for Desktop).
Another method, since you're working directly in SQL Server, is to use STPointFromText (geography Data Type) | Microsoft Docs to convert the coordinate pair into points in a view definition, not a physical table, with a shape column. The new column is a true shape and like SDEBINARY, MS Geometry is another kind of spatial type. It can be read and automatically be recognized as points if dragged into ArcMap. It's a little slower to draw since it performs these conversions every time you pan/zoom/redraw, but if you're just looking for a method for a limited number of points, it's not a deal breaker.
Once ArcMap could potentially read the source (originally lat/lon) through its shape column, you can use ArcGIS tools to append the data and map the attribute fields accordingly. Here's where it gets limiting for your use case. Every different spatial type is written differently in its values. You'll need Esri tools to convert between them. Running an Append tool (which can be scripted through ArcPy) will automatically make the conversion between source and destination behind the scenes with client-side processing.
