Hope you are doing well.
As I checked there isn't any app to import variants of existing products, regarding this we will need to implement custom script, so please check our queries :
# Do you have csv file of all the data that you want to upload?
#What would be types of attributes, please share the example.
Please let me know your reply so that I can estimate accordingly.
Please add me on Skype: cis.vincent and send reply here email@example.com so that I will check and we will move fruther.
I've helped a large number of stores migrate their data over to shopify. I've had many store owners who require product attributes to be stored and displayed on their product page... Because I found this to be a constant pain point for a lot of store owners I ended up building out an app resolve this issue. (See solution #2)
To answer your question (as of today), there is no native way to add custom product attribute fields into shopify.
HOWEVER, there is a rather easy solution, explained below:
1) download a metafield app. Metafield apps allow you to create metafields, or 'custom fields', to store customized fields such as dimensions, height, width, length, color, etc.
TIP: If you have A LOT of product attributes, find a metafield app that allows you EXPORT & IMPORT these fields. If you don't do this, you're pretty much screwed if you want to migrate/move/bulk edit your products after the fact.
Price Range: These apps range from $0-$20 on average.
If you want to display the product specifications on your product page, see below.
2) Download this app to create the technical specification charts/ product attribute charts / product specifications chart (whatever you call them!):
This app is the only app that allows you to create multiple charts for all of your products/ product types. When you create the chart, you can automatically have it pull data from shopify fields (like type, sku), metafields (like dimensions, color), and even allows you to provide manual entry.