I'll hazard a guess that you are reading data with VISA as a string. Then you want to convert that (probably with Scan From String) into a number. Then, you want to collect those numbers into an array.
We don't really have enough information to tell you the best way to do this. Here are some ideas.
If you are reading all the numbers from the instrument at once (e.g., a waveform from a scope), you might consider using "Spreadsheet string to array". Most scopes return the data as ASCII with commas between the values, so you'd wire a "," to the "delimiter" input of the "Spreadsheet string to array" function.
If, however, the instrument is returning a single point at a time, there are more questions. Do you want to do this continuously, or do you want t
o collect only a certain number of data points, and then process them as a block?
If the latter, then you'd just acquire each data point, convert it to a number, and then use output indexing on the for loop to create your array.
If the former, then you may want to store the data in a queue, and read it from the queue elsewhere in your program.
If you let us know what you want to do, we can probably provide more precise help.
Brian